As the infrastructure of underground utilities has aged the need to replace these underground utilities has grown. However, home and business owners do not like to have their landscaping and streets dug up during, the replacement of underground utilities. Thus, systems and methods for the replacement of underground utilities with minimal surface disruption have been developed. For example, horizontal directional drills are regularly used to install new and replace old utilities. Another technology widely used is a pit launched rod string pushing and pulling machine. These machines push a rod string, comprised of a series of rod string sections attached end-to-end, through the existing, pipeline from the launch pit to an exit point remote from the machine. The rod string sections may comprise solid rods, tubular members, or partially hollowed out sections of rod string. Rod string sections are added to the rod string as the rod string is pushed into the existing utility pipe.
One skilled in the art will appreciate that a downhole tool comprising a drill bit could be attached to the far end of the rod string to allow the rod string, to cut through the ground or an existing pipe. Once the far end of the rod string reaches the target location a different downhole tool may be attached to the far end of the rod string and used to burst the old utility pipe and guide the new replacement product pipe into the hole. The new product pipe to be installed may be connected to the downhole tool so that the new pipe follows the downhole tool back through the ground or old pipe to the launch pit. The machine grips the rod string and, using hydraulic cylinders, pulls the rod string, downhole tool, and new pipe toward the launch pit. The downhole tool may comprise a pipe bursting head configured to either burst or slice the old pipe and push it into the surrounding soil.
Oil rigs use gravity assisted slips to hold the drill string off the bottom of the bore, such as when tripping out to change the drilling tooling, or to provide torsional restraint when adding or removing the top from the string. Gravity assisted slips have a heavy walled outer slip bowl, slips, and jaws. The slip bowl is generally mounted on a structure that passes reaction forces to the ground. The slip bowl is ring shaped and has a conical inside surface running for its functional length; both ends of the bowl are open.
The drill pipe is disposed at cylindrical centerline of the slip bowl. The angle of the conical side relative to the centerline is on the order of five (5) to fifteen (15) degrees with a preferred angle of ten (10) degrees per side. Without the slips engaged with the rod string, the rod string is free to move in either direction along the axial centerline. Slips are generally thin walled segments having a conical surface on a first side and a cylindrical surface on a second side. The conical surface of the slip is configured to slide with low friction against the conical inner surface of the slip bowl. The cylindrical inner surface of the slip is intended to produce a high coefficient of friction against the matching cylindrical surface of the rod and may have a hardened and serrated finish intended to bite into the mating rod surface. The inner surface is the jaw and may be a replaceable component within the slip. There is generally a minimum of two slips and often there are more, up to a dozen.
Gravity causes the slips to drop into the tapered annular space between slip bowl and the rod. This causes friction between the rod and the slips. As the rod string moves down under the force of gravity the slip moves with it deeper toward the small diameter end of the slip bowl. Movement continues until at least two opposing slips apply normal forces to the slip bowl cone and the rod string. At this point the rod will be centered in the bowl and both the normal forces and the friction forces of the components rise quickly with slight distances of rod string travel.
The rod string and slips move deeper into the slip bowl until the friction forces on the rod string are equal in magnitude and opposite in direction to the weight (or other) forces pulling the rod string and causing movement. The rod will stop when the normal force around the bowl has caused the bowl to grow slightly within its elastic nature allowed by the geometry of all the components involved adjacent to and including, the slip bowl. The present invention provides a system to induce slip movement toward the small end of the slip bowl without requiring the force of gravity. Such a system clamps the rod string in either the vertical or horizontal orientation. The system of the present invention also allows the rod string to be clamped when it is being pushed in a direction that would typically cause the slips and jaws to release their grip. Additionally, the system allows the rod string to be clamped for resisting torsional loads when no tensile or compressive load exists on the rod string to cause the jaws to grip the is rod string. The powered gripping system of the present invention also provides a residual force on the rod string in the event the rod string is suddenly unloaded. The powered grip maintains the clamp load on the rod string and will cause the mass of the machine to absorb at least some of the stored energy to reduce the likelihood of the rod string traveling backwards through the machine unimpeded.